Giant Ragweed (Ambrosia trifida) is a dicot weed in the Asteraceae family. In Ontario this weed first evolved multiple resistance (to 2 herbicide sites of action) in 2011 and infests Soybean. Multiple resistance has evolved to herbicides in the Groups B/2, and G/9. These particular biotypes are known to have resistance to cloransulam-methyl, and glyphosate and they may be cross-resistant to other herbicides in the Groups B/2, and G/9.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

Greenhouse, and Laboratory trials comparing a known susceptible Giant Ragweed biotype with this Giant Ragweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on Group B, G/2, 9 resistant Giant Ragweed have not been reported to the site. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database. If you know anything about the mechanism of resistance for this biotype then please update the database.

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of multiple resistant Giant Ragweed from Ontario please update the database.

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Ontario have been instrumental in providing you this information. Particular thanks is given to Francois Tardif for providing detailed information.

Giant ragweed is a very competitive weed in row crop production and has been found to drastically reduce soybean yield. In 2008, giant ragweed was the first weed species with confirmed resistance to glyphosate in Canada. As of 2010 there were 48 locations with confirmed glyphosate resistant giant ragweed in Essex, Kent and Lambton counties. In addition, there was suspected resistance to cloransulam-methyl. The objectives of this research were (1) to conduct an expanded field survey on the distribution of glyphosate resistant giant ragweed in Ontario, (2) to determine the distribution of cloransulam-methyl resistant giant ragweed in Ontario, and (3) to determine the distribution of multiple resistant (glyphosate and cloransulam-methyl) giant ragweed in Ontario. In 2011 and 2012 giant ragweed seed was collected from 85 field sites in Essex (16), Kent (34), Lambton (23), Elgin (3), Middlesex (6), Lennox & Addington (1), Huron (1) and Brant (1) counties. In total there are 34 additional locations confirmed with glyphosate resistant giant ragweed in Ontario. There are 11 locations confirmed with cloransulam-methyl resistant giant ragweed and 5 locations with multiple resistance to both glyphosate and cloransulam-methyl. Glyphosate resistant giant ragweed has been found in 4 additional counties..

Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m-1 of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m-1 row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m-1 of cotton row for NAWF and 0.8 m-1 or 1.6 m-1 of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m-1 row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m-1 row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss..

Glyphosate resistant giant ragweed is an increasing problem in glyphosate resistant cropping systems in southwestern Ontario. The postemergence herbicides registered for use in soybean in Ontario do not provide consistent control of glyphosate resistant giant ragweed. There is limited research on the lowest effective rate of 2,4-D for the control of glyphosate resistant giant ragweed. Consequently, the objectives of this study were (a) to determine the efficacy of herbicides applied postemergence for the control of glyphosate resistant giant ragweed in glyphosate resistant soybean, and (b) to determine the lowest effective rate of 2,4-D for the control of glyphosate-resistant giant ragweed. Ten postemergence herbicide combinations and seven rates of 2,4-D were evaluated in field studies conducted in 2011 and 2012 at six locations confirmed with glyphosate-resistant giant ragweed. The post emergence herbicides evaluated did not provide acceptable/consistent control. Of the herbicides evaluated, glyphosate plus cloransulam-methyl provided 26% to 70% control 8 WAA of glyphosate resistant giant ragweed, which was the best of the herbicides combinations evaluated. The doses of 2,4-D required to reduce giant ragweed shoot dry weight by 50, 80 and 95% were 142, 310 and 1048 g a.e. ha-1, respectively..

The development of crops resistant to 2,4-D, dicamba, and glufosinate may provide new options for the management of glyphosate-resistant (GR) giant ragweed and other herbicide-resistant weeds. A fallow field study was conducted in 2011 and 2012 to determine the control of GR giant ragweed with 2,4-D and dicamba applied alone and in combination with glufosinate or fomesafen. Dicamba and 2,4-D tank-mixed with glufosinate or fomesafen provided the highest level of control at 10 or 20 days after application (DAA). At 30 DAA, all herbicide treatments provided >88% control of giant ragweed except glyphosate, glufosinate, and 2,4-D alone at 0.56 kg ae ha-1. Glyphosate, glufosinate, and 2,4-D alone at 0.56 kg ae ha-1 also had the highest number of giant ragweed plants (>5.8 plants m-2) and highest biomass (>19.2 g m-2). Contrast statements between 2,4-D and dicamba indicated no differences among treatments containing these herbicides. However, contrast analysis indicated that herbicides applied alone resulted in 56, 58, and 61% control while tank-mix combinations of 2,4-D or dicamba with glufosinate or fomesafen resulted in 86, 91, and 93% control, respectively. Herbicides applied alone also had more giant ragweed plants and biomass per m-2 than herbicides applied in tank-mix combinations. Tank-mixing combinations of 2,4-D and dicamba will be important for effective control of GR giant ragweed..

The germination ecology of Ambrosia artemisiifolia and A. trifida glyphosate susceptible biotypes sampled in marginal areas, was compared with that of the same species but different biotypes suspected of glyphosate resistance, common and giant ragweed, respectively. The suspected resistant biotypes were sampled in Roundup Ready® soybean fields. Within each weed species, the seeds of the biotype sampled in marginal area were significantly bigger and heavier than those of the biotype sampled in the soybean fields. A. artemisiifolia biotypes exhibited a similar dormancy and germination, while differences between A. trifida biotypes were observed. A. artemisiifolia biotypes showed similar threshold temperature for germination, whereas, the threshold temperature of the susceptible A. trifida biotype was half as compared to that of the resistant A. trifida biotype. No significant differences in emergence as a function of sowing depth were observed between susceptible A. artemisiifolia and suspected resistant A. trifida biotype, while at a six-cm seedling depth the emergence of the A. artemisiifolia susceptible biotype was 2.5 times higher than that of the A. trifida suspected resistant biotype. This study identified important differences in seed germination between herbicide resistant and susceptible biotypes and relates this information to the ecology of species adapted to Roundup Ready® fields. Information obtained in this study supports sustainable management strategies, with continued use of glyphosate as a possibility..